The Large Binocular Telescope will use two 8.408 meter (331-inch), F/1.142
primary mirrors to provide a collecting area equivalent to an 11.8 meter
(465-inch) circular aperture. By having both the primary mirrors on the same
mounting, the telescope will be able to achieve the diffraction-limited image
sharpness of a 22.8 meter aperture. These borosilicate honeycomb primary mirrors
are being fabricated at the Steward
Observatory Mirror Lab using E6 glass made by Ohara in Japan.

Included in the budget for the telescope project are the costs for a pair of
F/15 adaptive secondaries, each with 672 voice-coil actuators. Other focal
stations and features deemed scientifically important for future expansions
will be allowed in the optical and mechanical design of the telescope but not
initially implemented.

A pair of curved secondary mirrors and a pair of tertiary flat mirrors are required
to implement those focal stations. These smaller optics are moved in and out of the
lightpath by swing arms. The tertiary mirrors rotate to direct the light to several
central instrument locations (Prime focus cameras are not shown on the drawing to
the left).

The infrared F/15 secondaries are undersized so that the edge of the secondary mirror
serves as the aperture stop(s) for the telescope. This exit pupil can be reimaged and
baffled inside the infrared instruments to reduce the thermal emission from the structure
of the telescope. The infrared instrument will see the telescope pupil surrounded by a
cold sky background.